BY MICHELINA ANDREUCCI
The occurrence of natural diamonds is remarkable and important to Earth studies. They carry actual samples of mantle minerals from depths as great as 800km beneath the surface.
Diamond provides the perfect container for mantle minerals, isolating them from the high pressure and temperature reactions within the earth for geologic time scales.
Recent geological studies show that diamond can potentially form in any region on Earth provided the depth of the crust or the mantle is deep enough for the temperature to be high enough and provide high enough pressure. Most of the mantle is within the field of diamond stability.
The crust, which is normally too thin (usually less than 40km thick), to lie within this field, can do so only if it has been thickened by the geologic processes related to plate tectonics. Yet diamonds are very rare because the mantle has a relatively low abundance of carbon.
Temperature within the earth always rises with depth along a path known as the geothermal gradient, which is typically high enough for diamond growth at the necessary pressures. Yet, due to the relatively low quantity of carbon within the mantle, diamonds are very rare.
Furthermore, diamonds are not evenly distributed throughout the earth, and “mineable” quantities are found only in very unique geologic settings. Recently however, the graphite/diamond transition was revised to lower pressures providing for even greater storage of diamonds at shallower levels in the cratonic keel.
Cratons contain the oldest rocks within its host continent. Continental cratons are regions in which active tectonic plate processes ended long ago and are typically older than 2.5 billion years, from a geologic era known as the Archean, where diamondiferous kimberlites erupted through during the upliftment or eruption of the Earth.
Continental regions that long ago ceased participating in active plate tectonic processes such as rifting, mountain building, or subduction are known as continental cratons. They are easily defined by an absence of earthquake activity. Such regions have been leveled by long-term weathering and erosion, though they may be relatively recently uplifted, as is the case for southern Africa.
Most continents contain several cratons joined by younger crusts. The Kaapvaal Craton in Southern Africa is the oldest portions of the Archaean components of the larger Laurentia and Kalahari super cratons. The erosion of the earth’s surface through weathering of the ancient cratons has led to exposures of diamondiferous kimberlite, including the recent discovery of diamonds in Zimbabwe.
The erosion of ancient cratons has led to the weathering of surface exposures of kimberlite, and the release of diamonds. Without crustal uplift, diamonds remain trapped in geologic basins as in West Africa, Zimbabwe, and Brazil, where they can be panned like gold. Where the craton has been uplifted, diamonds released from their host rocks have been transported by rivers, such as the Orange River in South Africa, and longshore currents, such as the Benguela, along the continental shelf of the southern Atlantic Ocean.
Alluvial diamonds are recovered by placer and marine mining techniques that are very different from hard-rock kimberlite mining.
Until the early 20th Century, even the famed Koh-i-Noor, Hope, and Cullinan diamonds, were typically found in alluvial or surface deposits, almost by accident. The richness of the alluvial deposits of the Vaal and Orange Rivers of South Africa eventually led to the discovery of kimberlite and the famous workings around Kimberley, establishing kimberlite as the primary volcanic host of diamonds. To the list of early alluvial diamond producers (e.g., South Africa, Namibia, India, Congo, and Brazil) have been added other hard ground or primary rock countries (namely Botswana, Russia, Australia, and Canada), which have greatly increased worldwide production.
Due to Zimbabwe’s geological positioning and formation, much of the country’s area lies on the “Zimbabwe Archaean Craton” (within which the 2 575Billion-year old Great dyke lies, and acts as a strain-marker for the craton), said to be conducive to kimberlite deposits and suggesting hypothetically, that diamonds can originate practically everywhere in Zimbabwe.
The Great Dyke, a linear feature running from north-east to south-west of Zimbabwe, almost parallel with the Archaean craton which stretches from the north-east of the country to the south and western areas, extending into Botswana. The Dyke’s vast deposits, forms what is known as the “Orapa Kimberlite Track”, within which some of the world’s largest diamond mines are said to be found, including Botswana’s Orapa and Lethlekane Diamond Mines.
Excluding the River Ranch kimberlite mine close to the South African border, Diamond production in Zimbabwe prior to 2004, was limited to “chance” finds mostly during diggings for alluvial gold.
Between 1997 and 1998, Rio Tinto Zimbabwe discovered the Murowa Kimberlite cluster, and began mining operations in 2004, producing “typical African kimberlitic” diamonds, with an average value of US$65 per carat.
Zimbabwe’s great “Diamond Rush” began in June 2006, with the discovery of diamonds in Eastern Zimbabwe, where there is an estimated 70km kimberlite belt of diamonds extending from the Chiadzwa district of Marange to the Chimanimani Mountains, close on the border with Mozambique
Dr. Michelina Andreucci is a Zimbabwean-Italian researcher, and is a published author in her field. For Comments E-mail: inamanucci@gmail.com
About Author
